Footings play a crucial role in transferring the load of a structure to the ground safely and efficiently. Among the various types of footings used in construction, strip footing and strap footing are commonly used for different purposes. This blog explains these two types of footings in detail, highlights their differences, and discusses when and where each is suitable.
What is a Strip Footing?
A strip footing is a continuous strip of concrete that spreads the load of a wall or series of closely spaced columns along the length of the footing. It is primarily used for shallow foundations.
Key Features of Strip Footing:
- Shape and Design: It is a linear or rectangular foundation that runs under walls or rows of columns.
- Load Distribution: The footing distributes the load evenly across its length.
- Depth: Typically used for shallow foundations where the soil has sufficient bearing capacity near the surface.
- Material: Made of reinforced or plain concrete, depending on the load and soil conditions.
Applications of Strip Footing:
- Used in residential buildings and low-rise structures.
- Common for walls or rows of columns that don’t require deep foundations.
- Suitable for soils with good load-bearing capacity.
What is a Strap Footing?
A strap footing is a type of combined footing where two or more individual footings are connected by a beam (the strap). This design ensures the load is distributed evenly without overstressing the soil.
Key Features of Strap Footing:
- Shape and Design: Comprises two isolated footings connected by a strap beam.
- Load Distribution: The strap transfers load between footings to balance forces and prevent uneven settlement.
- Depth: Can be used for shallow or intermediate depths.
- Material: Typically made of reinforced concrete.
Applications of Strap Footing:
- Used when a column is located near a property boundary, and a single footing cannot be centred under the load.
- Common in commercial or industrial buildings where load distribution is critical.
- Suitable for soils with moderate to high load-bearing capacity.
Strip Footing vs Strap Footing: Key Differences
| Aspect | Strip Footing | Strap Footing |
|---|---|---|
| Design | Continuous strip under walls or rows of columns | Two isolated footings connected by a beam |
| Purpose | Distributes load evenly along the length | Balances load between two footings |
| Location of Columns | Suitable for evenly spaced loads | Used for columns near boundaries or uneven loads |
| Load Distribution | Uniform across the strip | Transfers load via the strap beam |
| Material Requirement | Requires less material than strap footing | Requires additional material for strap beam |
| Cost | More economical | Generally more expensive |
| Application | Low-rise buildings with continuous walls | Commercial or boundary-adjacent structures |
Advantages of Strip Footing
- Simple to design and construct.
- Cost-effective due to minimal materials required.
- Ideal for soils with good bearing capacity.
- Effective for evenly distributed loads.
Disadvantages of Strip Footing
- Not suitable for uneven loads or weak soils.
- Limited to shallow foundations.
Advantages of Strap Footing
- Ideal for columns near boundaries, where traditional footings cannot be placed centrally.
- Prevents differential settlement by balancing the load between footings.
- Suitable for heavier loads and uneven soils.
Disadvantages of Strap Footing
- More complex to design and construct.
- Requires more material, increasing costs.
- The strap beam does not contribute directly to load transfer.
When to Use Strip Footing vs Strap Footing
Strip Footing:
- Use when the structure has continuous walls or closely spaced columns.
- Suitable for residential or low-rise buildings.
- Best for soils with a good bearing capacity near the surface.
Strap Footing:
- Use for boundary-adjacent columns or where traditional footings cannot be placed directly under the load.
- Suitable for commercial buildings with uneven loads.
- Ideal for soils with varying load-bearing capacities.
IS Codes Considerations:
For Strip Footing:
- IS 456: 2000 – Code of Practice for Plain and Reinforced Concrete:
- Provides guidelines for the design and construction of reinforced concrete foundations, including strip footings.
- IS 1904: 1986 – Code of Practice for Design and Construction of Foundations in Soils: General Requirements:
- Offers general requirements for the design of shallow foundations, including strip footings.
- IS 6403: 1981 – Code of Practice for Determination of Bearing Capacity of Shallow Foundations:
- Specifies methods for calculating the bearing capacity of soils for shallow foundations like strip footings.
For Strap Footing:
- IS 456: 2000 – Code of Practice for Plain and Reinforced Concrete:
- Covers the design and detailing of strap beams and footings in reinforced concrete structures.
- IS 3370: 2009 – Code of Practice for Concrete Structures for Storage of Liquids:
- Although focused on liquid storage structures, its provisions for reinforced concrete design are relevant for strap footing beams.
- IS 1904: 1986 – Code of Practice for Design and Construction of Foundations in Soils: General Requirements:
- Provides guidelines for combined and strap footings.
- IS 2974 (Part 1): 1982 – Code of Practice for Design and Construction of Machine Foundations:
- Includes guidelines for foundations subjected to dynamic forces, where strap footings may also be applied.
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Conclusion🎯
Both strip footing and strap footing are essential in construction, each serving specific purposes based on structural needs and site conditions. Strip footings are straightforward and economical, making them ideal for small-scale projects, while strap footings are specialised and suited for complex load distribution scenarios.